Owing to physical and technical conditions, such as a limited homogeneity in the basic magnetic field and/or non-linearities of the gradient fields, the measurable volume in a magnetic resonance (MR) system is restricted in all three spatial directions. A scanning volume, what is known as a Field of View (FoV), is therefore limited to a volume in which the physical conditions mentioned above lie within specified tolerance ranges and therefore an image of the object to be examined which is true to the original is possible without significant local distortions. In other words, inhomogeneities in the basic magnetic field and non-linearities of spatially-encoding gradient fields are typically within a tolerance range in the field of view of the MR system, wherein the tolerance range is determined with respect to small distortions of MR data or for distortions which are not significant to applications.
From a geometrical perspective this field of view, in particular in the radial direction, i.e. in a transversal plane (for example in the x and y directions) perpendicular to a longitudinal axis (axial direction) of a tunnel or a tube of the MR system, is significantly smaller than the volume defined by the tunnel opening of the magnetic resonance system. In conventional magnetic resonance systems a diameter of the tunnel is by way of example 60 or 70 cm, whereas the diameter of the conventionally used field of view, in which the physical conditions mentioned above lie within the tolerance ranges, can be approximately 50 or 60 cm.
Acquired MR data can therefore exhibit distortions as a function of location. A distortion indicates a mismatch between a position of an image element in the MR data and the actual position of the image element in the object to be examined. In other words, the distortion describes a spatial imprecision in an MR image which is generated with the aid of MR data.
Many applications require high spatial precision, however, i.e. spatially precise imaging even outside of and adjoining the field of view: examples are a determination of a human attenuation correction for positron emission tomography (PET), MR-led interventions or applications in which spatially precise imaging methods, such as computed tomography (CT) or PET, are combined with MR methods.
The restriction that, in particular in the edge region of the tunnel of the MR system, comparatively severe distortions of the object being measured are possible, is conventionally avoided in the case of pure MRT imaging by arranging the corresponding test region of the object to be examined not at the edge of the tunnel but in a low-distortion region, for example as close as possible to the center of the tunnel, what is known as the isocenter of the magnetic resonance scanner. In conventional MR systems and in particular hybrid systems, such as a hybrid system comprising an MR system and a positron emission tomograph, what is known as an MR-PET hybrid system, it may, however, be desirable to determine structures in the subarea at the edge of the tube of the MR system as spatially precisely as possible as well. In an MR-PET hybrid system the human attenuation correction by way of example is critical. The human attenuation correction determines the intensity attenuation of the PET photons emitted after an interaction of positrons and electrons on their way to the detector through absorbent tissue, and corrects the received signal by precisely this attenuation. MR data is acquired for this which maps the complete anatomy of the object to be examined in the direction of the high-energy photons emitted by the positron emission tomography. This means that the anatomy of the object to be examined should also be captured as precisely a possible in the subarea at the edge of the tunnel of the hybrid system. In a patient to be examined the structures located in this subarea are primarily the arms by way of example.
Various correction algorithms are known in the prior art for correcting a distortion, in particular outside of the field of view, i.e. outside of the volume in which magnetic field inhomogeneity and non-linearity of the gradient field lie within specifications. Distortion-reduced MR data can be determined in this way. By way of example, a gradient distortion correction is proposed by S. Langlois et al. in “MRI Geometric Distortion: a simple approach to correcting the effects of non-linear gradient fields” (J. Magn. Reson. Imaging 1999, 9(6) 821-31) and by S. J. Doran et al. in “A complete distortion correction for MR images: I. Gradient warp correction” (Phys. Med. Biol. 2005 50(7) 1343-61). A correction of the basic magnetic field is proposed, moreover by S. A. Reinsberg et al. in “A complete distortion correction for MR images: II. Rectification of static-field inhomogeneities by similarity-based profile mapping” (Phys. Med. Biol. 2005 50(11) 2651-61).
The results of the proposed methods constitute comparatively complicated approaches, however, in particular for a distortion correction in the edge region. A subsequent correction may for example not be possible or only be possible to a limited extent. If, for example, the inhomogeneities in the basic magnetic field are so great that no clear frequency allocation can be ensured during the spatial encoding by means of gradient fields, it may be possible to correct errors resulting therefrom to only a limited extent after measurement.